MIL-HDBK-217 Part Stress & Part Count

This standardization handbook was developed by the US Department of Defense with the assistance of the military departments, federal agencies, and industry. The most widely known and used reliability prediction handbook is MIL-217. It is used by both commercial companies and the defense industry, and is accepted and known world-wide. It contains failure rate models for numerous electronic components such as integrated circuits, transistors, diodes, resistors, capacitors, relays, switches, connectors and more, see RAM Commander Library.
Reliability predictions are an important tool for making design trade-off decisions and estimating future system reliability. They are often used for making initial product support decisions such as how many spares are required to support fielded systems. Inaccurate predictions can lead to overly conservative designs and/or excessive spare parts procurement resulting in additional life cycle cost (LCC).
MIL-HDBK-217 handbook includes a series of empirically based failure rate models covering virtually all electrical/electronic parts under 14 separate operational environments, such as ground fixed, airborne inhabited, etc. There are two primary prediction approaches: the Part Stress technique and the Parts Count technique. As their names imply, the Part Stress technique requires knowledge of the stress levels on each part to determine their failure rates, while the Parts Count technique assumes average stress levels to provide an early design estimate of the failure rates. Typical factors used to determine a part's failure rate include a temperature factor (πT), a power factor (πP), a power stress factor (πS), a quality factor (πQ), and an environmental factor (πE) in addition to the base failure rate (λb). For example, the failure rate model for a resistor is as follows:

λResistor = λb πT πP πS πQ πE
Where λResistor is the estimated failure rate for the resistor in failures per million operating hours. The handbook does not include failure rate models for calculating dormant failure rates in non-operating conditions.
“Part Stress Analysis” (PSA) and “Part Count Analysis” (PCA) methods vary in degree of information needed to apply them. PSA requires greater amount of information and is used later in the design process when circuit structures become available. PCA only requires part quantities, quality levels and the application environment, and is therefore less accurate but useful during the early design phase and during the proposal formulation or “tentative device specification”. 
A similarity between PCA and PSA is that both prediction techniques use the same formulas. For PCA only estimated values are used while PSA uses calculated or measured values. There is a similarity between most part models. Generally speaking a failure rate formula will look like:
 λP = λbπEπQ

λ= Part failure rate
λ= base failure rate, dependent on temperature and applied stress
π = acceleration factors for the used environmental application and other parameters that will affect the part reliability 
πE = Environmental acceleration factor

πQ = quality acceleration factor

Environment and quality are used for most parts, while other p factors are part dependent. The base failure rate is usually expressed by a model addressing the influence of electrical and temperature stresses on a part. One of the most characteristic parts of many models is the relation between temperature and failure rate. These models use thermal stresses in a form related to the Arrhenius Law.

Other acceleration factors are modeled in terms of acceleration factor p. The data used to model these acceleration factors are mostly obtained from manufacturers data and field returned data. Using this method, it is possible to model the effects of using a component under certain environmental conditions, the effect of using specific methods of component quality screening, etc.
Ideally, the Parts Count technique is applied early in the design phase to determine that the predicted reliability is in the "ball park" with reliability requirements. As more detailed design information becomes available, such as detailed circuit schematics, the predictions should be refined to reflect actually applied component stress levels. This necessitates switching to the more detailed Part Stress reliability prediction methodology, which can result in significantly more labor hours for circuit analysis to compute the actual stress levels for each part application. Some companies simply use the Parts Count results as the final mean-time-between-failure (MTBF) estimates even though the results can be conservative because of the default stress levels assumed in the methodology, leading to sometimes costly decisions such as the procurement of additional spare assemblies.
Some companies impose circuit design rules such as component derating levels; however, they do not receive the benefit of this policy in their Parts Count reliability predictions because of the higher default stress levels assumed by MIL-HDBK-217. The way to correct this is to either switch to the full Part Stress method or perform a "Pseudo Part Stress" analysis that assumes average stress levels based on company design policies.
The two methods vary in the degree of information required for their completion. The Part Stress Analysis Method requires a greater amount of detailed information and is usually more applicable to the later design phase. By component stresses, the standard is referring to the actual operating conditions such as environment, temperature, voltage, current and power levels applied, for example. The MIL-217 standard groups components or parts by major categories and then has subgroups within the categories. For example, a “fixed electrolytic (dry) aluminum capacitor” is a subcategory of the “capacitor” group. Each component or part category and it's subgroups have a unique formula or model applied to it for calculating the failure rate for that component or part.
The Parts Count Analysis Method requires less information such as part quantities, quality level and application environment. It is most applicable during early design or proposal phases of a project.
For equipment operating in multiple environments the calculations should be applied to a portion of the equipment in each environment.
The MIL-217 standard provides tables for the component groups (same groups as the Parts Stress analysis) listing generic failure rates and quality factors for the different MIL-217 environments.
The Parts Count analysis does not factor in the numerous variables and uses worst case generic or base failure rates and pi factors.  The Parts Count Method will usually result in a higher failure rate or lower system reliability, that is providing a more conservative result than the Parts Stress Method would produce.

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